(a) Field of the Invention
The present invention relates to a thermo-sensitive infrared ray detector and, more particularly, to a thermo-sensitive infrared ray detector having a thermal isolation structure.
(b) Description of the Related Art
A thermo-sensitive infrared ray detector is generally used for detecting the temperature of an object by detecting infrared rays radiated from the object.
The thermo-sensitive infrared ray detector absorbs infrared rays radiated from the object at an infrared ray absorption film, installed therein and having an optical resonator structure, to convert infrared rays into heat. The heat generated by the conversion raises the temperature of a thermo-sensitive resistor, such as a bolometer film, forming a diaphragm having a micro bridge structure. The temperature of the object can be detected by the increase of the resistance of the thermo-sensitive resistor or bolometer film.
The thermo-sensitive infrared ray detector having such a thermal isolation structure generally involves a drift in the output signal due to a fluctuation of the ambient temperature, because it detects the infrared ray by measuring the temperature change of the bolometer film itself. The drift prevents an accurate measurement of the infrared ray by the thermo-sensitive infrared ray detector (hereinafter, may be referred to as simply xe2x80x9cinfrared ray detectorxe2x80x9d).
For suppression of the drift in the output signal of the infrared ray detector caused by the fluctuation of the ambient temperature, it may be considered that a temperature control unit is associated with the infrared ray detector. However, this raises the cost of the infrared ray detector. A technique obviating the use of the temperature control unit is described in, for example, Patent Publications JP-A-11-248530 and -10-227689.
FIG. 1 shows the structure of the infrared ray detector described in JP-A-11-248530, and FIG. 2 shows the schematic circuit diagram of the amplifier disposed therein. The infrared ray detector includes an array of pixels formed on a substrate 82, each of the pixels including a metal bolometer 80, and a resistor 83 made of a material same as the material of the metal bolometer 80 and embedded in the substrate 82. The thermal isolation structure wherein the metal bolometer 80 is supported by a pair of struts 81 for thermal isolation of the metal bolometer 80 from the substrate 82 allows the metal bolometer 80 to change the resistance thereof upon irradiation of an infrared ray. On the other hand, the resistor 83 embedded in the substrate 82 exhibits a little temperature change upon the irradiation of the infrared ray.
The operational amplifier or inverting amplifier 84 having the resistor 83 as an input resistance (Rs) and the metal bolometer 80 as a feedback resistor (Rf) outputs a voltage signal representing a resistance ratio RF/RS. This configuration, wherein the resistance of the resistor 83 is used as a reference value, allows the cancellation of the fluctuation of the ambient temperature from the output of the metal bolometer 80, whereby installation of a temperature control unit is obviated in the infrared ray detector. In addition, since both the metal bolometer 80 and the resistor 83 are formed by using a thin film technique, the difference in the physical property between the metal bolometer 80 and the resistor 83 can be made minimum to thereby improve the accuracy of the measurement.
FIG. 3 shows a read circuit used in the infrared ray detector described in JP-A-10-227689, wherein the read circuit includes a chopper amplifier. The infrared ray detector includes in a single pixel a first thermo-sensitive resistor 101 and a second thermo-sensitive resistor 102 which constitutes a dummy resistor. The chopper amplifier includes first switch 104a, second switch 104b, third switch 104c, a capacitor 106 and an inverter 107.
The first thermo-sensitive resistor 101 and the dummy resistor 102 are connected to a current mirror 103, whereby the same current flows through the first thermo-sensitive resistor 101 and the dummy resistor 102. After the first switch 104a is activated (or closed) while both the resistors 101 and 102 pass the current, the output signal on a first node 105a is transmitted to one of the terminals of the capacitor 106, the other of the terminals of which is connected to the input of the inverter 107 through a second node 105b. By supplying a clock signal to activate the second switch 104b when the capacitor 106 receives the signal, the input and the output of the inverter are short-circuited, determining the operational point of the amplifier.
Thereafter, the first and second switches 104a and 104b are made open and the clock signal is supplied to the third switch 104c to activate the same, whereby the signal on the second node 105b is transmitted to the capacitor 106. The third node 105c allows a potential equal to a potential difference between the first node 105a and the second node 105b to be delivered to the third node 105c. The potential difference between the first node 105a and the second node 105b corresponds to the temperature rise which corresponds to the amount of the infrared ray irradiation. Thus, the signal on the third node 105c is delivered from the amplifier through the inverter 107.
FIG. 4 shows the infrared ray detector described in JP-A-10-227689, wherein a thermo-sensitive resistor 121 including a first bolometer film 131 and a dummy resistor 122 including a second bolometer film 132 are juxtaposed on a silicon substrate 123 in each pixel. The first bolometer film 131 is thermally isolated from the silicon substrate 123 by a cavity 126 disposed therebetween and formed by using a micro-machining technique, whereby the first thermo-sensitive resistor 121 is susceptible to a temperature rise caused by infrared ray irradiation. The second bolometer film 132 has a shape and dimensions similar to the shape and dimensions of the first bolometer film 131, and is located on the silicon substrate 123 via a support plate 124.
Both the first and second bolometer films 131 and 132 have similar temperature coefficient of resistances (TCR) so that the fluctuation of the ambient temperature does not cause any substantial change of the output of the infrared ray detector. The support plate 124 of the second bolometer film 132 may have small thickness as shown in FIG. 4 or may have a larger thickness as shown in FIG. 5, which shows a modification of the infrared ray detector of FIG. 4.
In the infrared ray detector shown in FIG. 4 or 5, if the second bolometer film 132 is irradiated by an infrared ray, the heat generated by the infrared ray irradiation in the second bolometer film 132 is readily transferred to the silicon substrate 123 acting as a heat sink, whereby the resistance of the second bolometer film 132 is not changed by the infrared ray irradiation. More specifically, the second bolometer film 132 is susceptible only to the fluctuation of the ambient temperature whereas the second bolometer film 131 is susceptible to both the infrared ray irradiation and the fluctuation of the ambient temperature. By combining this configuration with the signal read circuit of FIG. 2, while a DC output voltage component is made constant irrespective of the ambient temperature, the signal component caused by the infrared ray irradiation is superimposed on the DC output voltage component.
In both the conventional infrared ray detectors, as described above, although the metal bolometer 80 and the first bolometer film 131 are thermally isolated from the substrates 82 and 123, respectively, the dummy resistor 83 and the second bolometer film 132 are disposed substantially directly on the substrate acting as a heat sink. This configuration allows the fluctuation of the ambient temperature to be cancelled by using the output difference as described above; however, it is difficult to solve the problem that the self-heating of the resistors or bolometer films caused by a bias current may cause a fluctuation of the output voltage. This problem is detailed below.
The bias current is generally used in an on-chip read circuit installed in the infrared ray detector for reading the output signal from an array of pixels each including bolometer films (Refer to xe2x80x9cThe 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX.xe2x80x9d 25-29, 1995 by Tanaka et al.).
If a pulse bias current passes through an infrared ray detector having a thermal isolation structure, the temperature of the bolometer itself rises sharply due to the Joule heat by self heating, and then falls toward the original temperature upon cut-off of the pulse bias current. The temperature difference of the bolometer in this case assumes several tens of degrees of Celsius. On the other hand, if an object having a temperature difference of about 0.1xc2x0 C. with respect to the ambient temperature is detected by an infrared ray camera including an optical system having a F-number of F/1, the temperature of the bolometer rises by about 0.2 milli-degree Celsius (mxc2x0 C.). In short, a bolometer-type infrared ray camera driven by a pulse bias current operates on a small signal component superimposed on a large self-heating component.
In the following description, the infrared ray detector described in JP-A-11-248530 is referred to. When the bolometer is subjected to infrared ray irradiation, the bolometer exhibits a change in the resistance thereof. The temperature dependency of the resistance of a metal is generally expressed by the following formula:
R=R0exp(xcexaMT)xe2x80x83xe2x80x83(1) 
wherein R0 is a constant depending on the geometry of the bolometer, and xcexaM is the temperature coefficient of resistance (TCR) of the metal and thus determined by the species of the metal. In view of this formula, if the temperature TA of the substrate 82 fluctuates, the temperature TB of the bolometer 80 having a resistance of RB and the temperature TR of the resistor 83 having a resistance of RR are expressed as the following formulas:                                           T            R                    =                                    T              A                        +                                          ∑                                  i                  =                  1                                ∞                            ⁢                              xe2x80x83                            ⁢                              Δ                ⁢                                  xe2x80x83                                ⁢                                  T                  Ai                                ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              t                                                  t                          i                                                                                      )                                                                                      ;                            (        2        )                                                      T            B                    =                                    T              A                        +                                          ∑                                  i                  =                  1                                ∞                            ⁢                              xe2x80x83                            ⁢                              Δ                ⁢                                  xe2x80x83                                ⁢                                  T                  Ai                                ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              t                                                                              t                            i                                                    +                                                      τ                            th                                                                                                                )                                                                        +                          Δ              ⁢                              xe2x80x83                            ⁢                              T                OBJ                                      +                          Δ              ⁢                              xe2x80x83                            ⁢                              T                J                                                    ;                            (        3        )                                                      Δ            ⁢                          xe2x80x83                        ⁢                          T              OBJ                                =                                                    I                in                                            G                th                                      ⁢                          (                              1                -                                  exp                  ⁡                                      (                                          -                                                                        τ                          f                                                                          τ                          th                                                                                      )                                                              )                                      ;        and                            (        4        )                                                      Δ            ⁢                          xe2x80x83                        ⁢                          T              J                                =                                                    V                B                2                                            R                B                                      ⁢                          1                              G                th                                      ⁢                          (                              1                -                                  exp                  ⁡                                      (                                          -                                                                        τ                          ro                                                                          τ                          th                                                                                      )                                                              )                                      ,                            (        5        )            
wherein xcex94 TAi is a fluctuation of the ambient temperature having a period ti, xcfx84th is a thermal time constant of the bolometer having a thermal isolation structure, xcex94 TOBJ is a temperature rise of the bolometer having the thermal isolation structure caused by the thermal radiation Iin(W) from the object, xcex94 TJ is a temperature rise of the bolometer having the thermal isolation structure caused by Joule heat, VB is a bias voltage, Gth is the thermal conductance of the thermal isolation structure, xcfx84r0 is a time length of the bias pulse, and xcfx84f is a frame time. The relationship xcfx84r0 less than  less than xcfx84th in the assumption of the literature by Tanaka et al. as recited above provides an approximation as follows:                                           Δ            ⁢                          xe2x80x83                        ⁢                          T              J                                =                                                    V                B                2                                            R                                  B                  -                                                      ⁢                                          τ                ro                                            C                th                                                    ,                            (        6        )            
wherein Cth is a heat capacity of the temperature sensor including the metal bolometer 80 in the thermal isolation structure shown in FIG. 1. Since the temperature dependency of the resistance of the metal is expressed by formula (1), the output signal Vout of the amplifier shown in FIG. 2 is expressed as follows:                                           V            out                    ∝                                    R              F                                      R              S                                      =                                            R              B                                      R              R                                =                                                    R                BO                                            R                RO                                      ⁢                                          exp                ⁡                                  (                                                                                    κ                        B                                            ⁢                                              T                        B                                                              -                                                                  κ                        R                                            ⁢                                              T                        R                                                                              )                                            .                                                          (        7        )            
Assuming an ideal case wherein the temperature coefficients of the resistance are same between the temperature sensors, i.e., xcexaB=xcexaR, the output signal Vout is expressed by:                               V          out                ∝                                            R              BO                                      R              RO                                ⁢                                    exp              ⁡                              (                                                      κ                    B                                    ⁡                                      (                                                                  T                        B                                            -                                              T                        R                                                              )                                                  )                                      .                                              (        8        )            
It is to b e noted in formulas (2) and (3) that the fluctuation period ti of the drift by the ambient temperature in the second term is generally far greater than the thermal time constant xcfx84th. Thus, the following relationship:
TBxe2x88x92TR≈xcex94TOBJ+xcex94TJxe2x80x83xe2x80x83(9) 
is obtained from equations (2) and (3).
In addition, since the temperature change xcex94 TOBJ of the bolometer 80 caused by the thermal radiation from the object is far smaller than the temperature rise xcex94 TJ caused by the Joule heat, the relationship (9) is replaced by:                                                         T              B                        -                          T              R                                ≈                      Δ            ⁢                          xe2x80x83                        ⁢                          T              J                                      =                                            V              B              2                                      R              B                                ⁢                                                    τ                ro                                            τ                th                                      .                                              (        10        )            
As understood from the relationships (8) and (10), the output voltage Vout depends strongly on the bias voltage VB. Since the bias voltage VB is liable to change depending on the temperature fluctuation of the read circuit, the temperature difference TBxe2x88x92TR changes in proportion to VB2 so long as the metal bolometer 80 has a thermal isolation structure and the resistor 83 is formed on the substrate 82, whereby the output voltage changes significantly irrespective of the circuit arrangement of FIG. 2. This problem is common to the infrared ray detector described in JP-A-10-227689.
In addition, in both the conventional infrared ray detectors, incorporation of the thermo-sensitive resistor and the dummy resistor in each pixel lowers the effective opening ratio expressed by a ratio of the infrared-ray sensitive area to the total pixel area, thereby decreasing the sensitivity to the infrared ray.
It is therefore an object of the present invention to provide a thermo-sensitive infrared ray detector which is capable of suppressing the drift in the output signal of the detector caused by the ambient temperature or the self heating of the resistors in the detector and has a higher sensitivity to infrared rays without incorporating a temperature control unit, such as a Peltier element.
The present invention provides a thermo-sensitive infrared ray detector including a substrate, first and second sections formed on said substrate and including first and second thermo-sensitive resistors, respectively, said first and second thermo-sensitive resistors having similar dimensions and thermally isolated from one another and from said substrate, and a shield member for shielding said second thermo-sensitive resistor against an infrared ray.
In accordance with the thermo-sensitive infrared ray detector of the present invention, the difference between the output signals from both the thermo-sensitive resistors includes substantially only a signal component representing the temperature change caused by the infrared ray incident onto the infrared ray detector, with the fluctuation of the ambient temperature being cancelled in the difference. Thus, an accurate measurement for the amount of infrared ray radiation can be obtained.